Recently, as the sizes and thicknesses of still cameras and video movie cameras have been further reduced, the size and thickness of a barrier unit, which is used to protect their lens, have also been reduced year by year.
Hereinafter, a conventional barrier unit will be described as an example with reference to FIGS. 6 through 9.
FIGS. 6 and 7 are exploded perspective views respectively illustrating the upside and downside of respective members of a conventional barrier unit.
The conventional barrier unit includes a cap 1, first and second blades 2 and 3, a base 4, first and second rings 5 and 6, and first and second springs 7 and 8.
The first and second blades 2 and 3 are pivoted to the base 4 by fitting pivots 9 and 10 on the base 4 into pivot holes 15 and 16, respectively. By getting hooks 11, which are elastic members that form integral parts of the cap 1, engaged with hook stoppers 12 of the base 4, the cap 1 is fitted to the base 4 so as to sandwich the first and second blades 2 and 3 between them and to leave a predetermined clearance that is wide enough to let those barrier blades 2 and 3 rotate freely there.
The cap 1 has a window 13. The first and second rings 5 and 6 are fitted rotatably to the cylindrical portion 14 of the base 4. The cam pins 19 and 20 of the first ring 5 are engaged into elongate holes 17 and 18 of the first and second blades to turn the blades.
The first spring 7 is extended between respective spring hooks 21 and 22 of the first and second rings 5 and 6. Likewise, the second spring 8 is extended between respective spring hooks 23 and 24 of the second ring 6 and the base 4.
The second ring 6 has a ring projection 25, which is fitted into a notched portion 26 of the first ring 5, thereby assembling the first and second rings 5 and 6 together.
The base 4 also has a window 27.
The second ring 6 has a cam portion 28, which is biased and driven by driving means (not shown) to the direction in which the first and second blades 2 and 3 close, as indicated by the arrow B.
The first and second rings 5 and 6 are combined together with the first spring 7. As the second ring 6 is driven in the closing direction (as indicated by the arrow B) by the cam portion 28, the first and second blades 2 and 3 are closed. And even if the second ring 6 can afford to further turn in the same closing direction after that, the first spring 7 expands to prevent the second ring 6 from going too far with respect to the first ring 5.
FIG. 8 is a schematic representation illustrating the opened position of the first and second blades 2 and 3 in the conventional barrier unit. On the other hand, FIG. 9 is a schematic representation illustrating how the first and second blades 2 and 3 in the opened position will backlash. FIGS. 8 and 9 illustrate the arrangement of the respective members in the opened position as viewed in the direction indicated by the arrow A in FIGS. 6 and 7.
As shown in FIG. 8, the second ring 6 is biased by the second spring 8 to the opening direction as indicated by the arrow C. As a result, the ring projection 25 of the second ring 6 presses the notch 26 of the first ring 5, thereby driving the first ring 5 in the direction indicated by the arrow C. In the meantime, the cam pin 19 also presses the elongate hole 17 and biases the first blade 2 to the opening direction. The first blade 2 is pressed against and brought into contact with the stopper 29 and stops turning there. The stopper 29 is arranged on the base 4. When the first blade 2 stops this way, the first ring 5 stops turning and the position of the other cam pin 20 is determined.
On the other hand, as the first ring 5 is driven in the direction indicated by the arrow C, the cam pin 20 also presses the elongate hole 18 and biases the second barrier blade 3 to the opening direction. When the position of the cam pin 20 is determined, that of the second blade 3 is also determined accordingly. Another stopper 30 is provided for the second blade 3 and is also arranged on the base 4.
Another exemplary conventional barrier unit is disclosed in Patent Document No. 1, for example.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-258120